F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

Cement 19
composition of Portland cement is, typically: lime 60-65%, silica 18-25%,
alumina 3-8%, iron oxide (Fe20 3) 0.5-5%. Different types of Portland
cement are obtained by varying the relative proportions of these four
predominant chemical compounds, and by grinding the clinker to different
degrees of fineness. The two most common types of cement [3] are
ordinary Portland cement and rapid-hardening Portland cement, which are
both covered by BS 12.
When cement in mixed with water, the various compounds of the cement
begin to react chemically with the water. For a short time, this cementwater
paste remains plastic, and it is possible to disturb it and remix
without harmful effects, but as the chemical reactions continue, the paste
begins to stiffen, or set. The arbitrary beginning and the ending of the
period of setting are called the initial set and the final set. The definition of
the stiffness of the paste which is to be regarded as set is necessarily vague.
In BS 12, for example, the initial setting time is defined as the interval
between the time when water is added to the cement and the time when the
paste will just withstand a prescribed pressure. Similarly, the final setting
time is defined as the interval between the time when water is first added
and that when the paste has further stiffened to be able to withstand a
higher prescribed pressure. In the test for setting times, a mix of standard
consistence is used: BS 12 defines standard consistence as that of a cement
paste to which such an amount of water has been added that, at a
prescribed time after adding water, the paste is just able to withstand a
prescribed pressure. The water content of such a paste, expressed as a
percentage of the dry weight of the cement, is usually from 26 to 33%.
If the cement sets too rapidly, the concrete will stiffen too quickly for it
to be transported and properly placed in the moulds or formwork; if it sets
too slowly, it might delay the use of the structure through insufficient
strength. BS 12 specifies that the initial setting time must be at least 45
minutes and the final setting time at most 10 hours. In practice, the rate of
setting is controlled by adding about 7-8% of gypsum to the clinker as it
is being ground. This gypsum is sometimes called a retarder, because it
retards the setting of the cement.
After the paste has attained final set, it continues to hydrate and increase
in rigidity and strength; this process is called hardening. It is important to
understand that setting and hardening are the result of chemical reactions,
in which water plays an important part; it is not just a matter of the paste
drying out. In the absence of moisture, these chemical reactions stop; in
the presence of moisture they may continue for many years so that the
hardened paste continues to gain strength. The setting and hardening of
the cement paste is accompanied by the liberation of heat, called the heat
of hydration, which for ordinary Portland cement averages about 300 kJ/kg
at 7 days, rising to about 340 kJ/kg at 28 days. In mass concrete
construction such as in dams, where it is difficult for this heat to escape, it
may become necessary to control the rise in temperature of the concrete by
using low-heat Portland cement (BS 1370), for which the heat of hydration
does not exceed 250 kJ/kg at 7 days and 300 kJ/kg at 28 days.
The rate of hardening increases with the fineness to which the cement
has been ground. In practice, fineness is defined by the specific surface of